Finite Element Analysis (FEA) is routinely used to perform fatigue analyses of cardiovascular stents. For the case of balloon expandable stents, this means modeling the crimping of the stent onto the delivery balloon, the expansion and recoil of the stent as would occur during deployment and finally the simulation of fatigue deformations. Fatigue deformations typically involve uniform radial pulsatile loading and/or bending of the stent between two different radii of curvature. A safety factor is determined by computing the alternating and mean stresses for the given cyclic loading conditions and comparing them to allowable material limits using a Goodman-type approach.
A simple method for determining the safety factor is to compute the alternating and mean stresses from the principle stress components for the minimum and maximum extremes of the cyclic loading conditions. The ratios of the alternating stress and endurance limit and the mean stress and the ultimate strength are combined and equated to the reciprocal of the safety factor, N.
This equation is evaluated for each integration point in the model. But as was discussed in our most recent ASTM F04.30.06 Fatigue to Fracture task group, this approach has several shortcomings.
An alternative approach is to use the individual stress components with respect to the global coordinate system for the two fatigue conditions and compute the individual alternating and mean stress components according to
These individual alternating and mean stress components are used to compute an effective alternating and an effective mean stress using
Subsequently, the effective alternating and mean stress values are used to compute a safety factor according to
An alternative approach would be to use the three principle stress components and compute the three alternating and mean principle stresses and then compute the effective stress according to the Gough–Pollard model; however, this approach still does not account for the effect of rotations in stress space on the mean and effective stress.
A more rigorous approach is to compute the alternating and mean stress components according to an element local coordinate system. Principal values of the stress tensor and associated maximum planes can then used in subsequent fatigue analysis.
Consistency and careful comparison to experimental data are indicated as large deformations and non proportional loading are common to most if not all implantable medical devices.